1. Field of the Invention
The present invention provides a downhole surge pressure reduction apparatus for use in the oil well industry. More particularly, the invention provides a surge pressure reduction apparatus that is run into a well with a pipe string or other tubular to be cemented and facilitates the cementing by reducing surge pressure and inner well sediments during run-in.
2. Background of the Related Art
In the drilling of a hydrocarbon well, the borehole is typically lined with strings of pipe or tubulars (pipe or casing) to prevent the walls of the borehole from collapsing and to provide a reliable path for well production fluid, drilling mud and other fluids that are naturally present or that may be introduced into the well. Typically, after the well is drilled to a new depth, the drill bit and drill string are removed and a string of pipe is lowered into the well to a predetermined position whereby the top of the pipe is at about the same height as the bottom of the existing string of pipe (liner). In other instances, the new pipe string extends back to the surface of the well casing. In either case, the top of the pipe is fixed with a device such as a mechanical hanger. A column of cement is then pumped into the pipe or a smaller diameter run-in string and forced to the bottom of the borehole where it flows out of the pipe and flows upwards into an annulus defined by the borehole and pipe. The two principal functions of the cement between the pipe and the borehole are to restrict fluid movement between formations and to support the pipe.
To save time and money, apparatus to facilitate cementing are often lowered into the borehole along with a hanger and pipe to be cemented. A cementing apparatus typically includes a number of different components made up at the surface prior to run-in. These include a tapered nose portion located at the downhole end of the pipe to facilitate insertion thereof into the borehole. A check valve at least partially seals the end of the tubular and prevents entry of well fluid during run-in while permitting cement to subsequently flow outwards. Another valve or plug typically located in a baffle collar above the cementing tool prevents the cement in the annulus from back flowing into the pipe. Components of the cementing apparatus are made of plastic, fiberglass or other disposable material that, like cement remaining in the pipe, can be drilled when the cementing is completed and the borehole is drilled to a new depth.
There are problems associated with running a cementing apparatus into a well with a string of pipe. One such problem is surge pressure created as the pipe and cementing apparatus are lowered into the borehole filled with drilling mud or other well fluid. Because the end of the pipe is at least partially flow restricted, some of the well fluid is necessarily directed into the annular area between the borehole and the pipe. Rapid lowering of the pipe results in a corresponding increase or surge in pressure, at or below the pipe, generated by restricted fluid flow in the annulus. Surge pressure has many detrimental effects. For example, it can cause drilling fluid to be lost into the earth formation and it can weaken the exposed formation when the surge pressure in the borehole exceeds the formation pore pressure of the well. Additionally, surge pressure can cause a loss of cement to the formation during the cementing of the pipe due to formations that have become fractured by the surge pressure.
One response to the surge pressure problem is to decrease the running speed of the pipe downhole in order to maintain the surge pressure at an acceptable level. An acceptable level would be a level at least where the drilling fluid pressure, including the surge pressure is less than the formation pore pressure to minimize the above detrimental effects. However, any reduction of surge pressure is beneficial because the more surge pressure is reduced, the faster the pipe can be run into the borehole and the more profitable a drilling operation becomes.
The problem of surge pressure has been further addressed by the design of cementing apparatus that increases the flow path for drilling fluids through the pipe during run-in. In one such design, the check valve at the downhole end of the cementing apparatus is partially opened to flow during run-in to allow well fluid to enter the pipe and pressure to thereby be reduced. Various other paths are also provided higher in the apparatus to allow the well fluid to migrate upwards in the pipe during run-in. For example, baffle collars used at the top of cementing tools have been designed to permit the through flow of fluid during run-in by utilizing valves that are held in a partially open position during run-in and then remotely closed later to prevent back flow of cement. While these designs have been somewhat successful, the flow of well fluid is still impeded by restricted passages. Subsequent closing of the valves in the cementing tool and the baffle collar is also problematic because of mechanical failures and contamination.
Another problem encountered by prior art cementing apparatus relates to sediment, sand, drill cuttings and other particulates collected at the bottom of a newly drilled borehole and suspended within the drilling mud that fills the borehole prior to running-in a new pipe. Sediment at the borehole bottom becomes packed and prevents the pipe and cementing apparatus from being seated at the very bottom of the borehole after run-in. This misplacement of the cementing apparatus results in difficulties having the pipe in the well or at the wellhead. Also, the sediment below the cementing apparatus tends to be transported into the annulus with the cement where it has a detrimental effect on the quality of the cementing job. In those prior art designs that allow the drilling fluid to enter the pipe to reduce surge pressure, the fluid borne sediment can fowl mechanical parts in the borehole and can subsequently contaminate the cement.
There is a need therefore for a cementing apparatus that reduces surge pressure as it is run-into the well with a string of pipe. There is a further need, for a cementing apparatus that more effectively utilizes the flow path of cement to transport well fluid and reduces pressure surge during run-in. There is a further need for a cementing apparatus that filters sediments and particles from well fluid during run-in.